In civil aviation a pressurized cabin is understood to be a configuration of a passenger cabin, cockpit and cargo compartment for high-flying airplanes which, under these life-threatening conditions per se, facilitates a flight without the use of oxygen masks for people. This is achieved by an artificially changed air pressure, which is increased with respect to the surroundings, within the cabin. The pressurized cabin is not hermetically sealed, but fresh air is steadily supplied and a part of the stale air is discharged from the pressurized cabin. The required supply air is supplied into the airplane by means of an air conditioner, so-called packs. In doing so, a relative positive pressure prevails within the cabin; therefore, the internal pressure within the cabin is above the external pressure of the surroundings. In order to not unnecessarily load the cabin structure, a predetermined maximum differential pressure between the internal pressure and external pressure is set.
Here, pressure regulation within the pressurized cabin is substantially effected by means of the valves mentioned below: outflow valves, negative pressure relief valves and safety valves.
The pressure in the pressurized cabin is controlled by means of the outflow valves, which in turn are controlled by a pressure controller, safety valves being additionally provided, by means of which pressure can be released in case of emergency. In the process, the outflow valves control the outflow of the air from the cabin and thus maintain the predetermined positive pressure in the cabin.
A system for controlling the internal pressure of the cabin is described in DE 60 2004 003 946 T2, for example.
When the airplane is descending, wherein the external pressure rises, the internal pressure in the pressurized cabin is usually increased at the same time so that the internal pressure is always above the external pressure. In case of a too fast descent or in case of failure of the air conditioner during the descent, it may happen that the external pressure exceeds the internal pressure (reversal of the differential pressure).
In order to prevent the pressurized cabin from collapsing and therefore to relieve the negative pressure existing in the pressurized cabin, the negative pressure relief valves open, which respectively may be embodied as a spring-loaded flap, for example, which opens inwards. Opening the negative pressure relief valves results in a sudden increase of pressure in the pressurized cabin, which the pressure controller tries to compensate in that it adjusts the outflow valves to an opened position, in which the respective flap of the outflow valves has an angle of about 90 degree to the air inflow direction at the outside of the fuselage. In the 90 degree position of the flaps of the outflow valves a part of the respective flap protrudes into the airflow. Depending on the design, a more or less strong air turbulence lowering the static external pressure in the region behind the flap results behind the flaps. Thereby air is sucked out of the pressurized cabin by the outflow valves. However, this additional airflow from the outflow valves has in turn to be compensated by the negative pressure relief valves to prevent a too great drop of the internal pressure in the pressurized cabin. As a consequence, the negative pressure relief valves have to be designed comparatively large, which in turn has a negative effect on the strength of the fuselage in the area of the negative pressure relief valves and results in increased costs of the negative pressure relief valves respectively, since they have to be designed correspondingly more stable, as being larger.